1. Field of the Invention
The present invention relates generally to line printers with interchangeable type carriers and, more particularly, to a system for discriminating the kind of type carrier loaded on such a line printer.
2. Description of the Prior Art
As is well known, there are a variety of type carriers, such as type belts, type trains and type drums. The present invention can be applied to all sorts of type carriers. However, for purposes of explanation, the present invention is described with reference to the type belt carrier.
As is well known in the art, type belts are classified according to the number of characters provided thereon, the distance between adjacent types (hereinafter referred to as "the type pitch"), the size of the types, and the type code. However, the present invention is described with reference to the number of characters on the type belt and, in one modification, to the type pitch.
In the case of a 48-character type belt, the type belt may have a plurality of type sets, each comprising 48 characters. On the type belt, type marks are provided below the respective types and a synchronizing mark indicating the beginning of the type set is also provided.
In order to discriminate the type belt being used, a method has been proposed where the number of characters between synchronizing marks is counted. However, such a discrimination method exhibits deficiencies in that the discrimination time is relatively long, it is necessary to provide a counter having a count capacity equal to or greater than the maximum number of characters, and the arrangement of the device to implement such a method is intricate. This method was disclosed in Mayo, R. F., "Print Element Character Set Status Logic", IBM Technical Disclosure Bulletin, Volume 16, No. 6, November 1973, pp. 1937-1938.
Another method for discriminating the type belt being used is disclosed in U.S. Pat. No. 3,899,968 to McDevitt, issued Aug. 19, 1975. This method uses a plurality of synchronizing marks at the beginning of the character set on the type belt to provide an identification tag. The identification tag is read from the belt into a shift register and then provided to an identification register, whose output is compared in a comparator with the verification tag provided by a verification register. Printing is allowed to take place only when the verification tag is identical to the identification tag. This method exhibits deficiencies in that a considerable amount of logic circuitry is needed to provide the desired identification function due to the use of the relatively complex binary coded identification tag.
Other prior art methods which provide an indication of the beginning of the font of a type belt or type drum are found in, respectively, U.S. Pat. No. 3,875,545 to Curtiss, issued Apr. 1, 1975, and U.S. Pat. No. 3,117,514 to Doersam, issued Jan. 14, 1964. However, neither of these systems disclose a type belt identification capability.